The present invention relates generally to optical systems. More specifically, the present invention relates to a system for the fabrication of periodic or aperiodic structures in an optical substrate.
Optical fibers are long, thin strands of very pure glass which are used to transmit light signals over long distances. Each optical fiber typically has three parts: a core, a cladding, and a buffer coating. The core is the thin glass center of the fiber where the light travels. The cladding is the outer optical material surrounding the core that reflects the light back into the core because it has an index of refraction less than that of the inner core. The buffer coating is a polymer coating that protects the fiber from damage and moisture. Large numbers of these optical fibers can be arranged in bundles to form optical cables.
A fiber grating is a periodic or aperiodic perturbation of the effective absorption coefficient and/or the effective refractive index of an optical waveguide. It can reflect a predetermined range of wavelengths of light incident on the grating, while passing all other non-resonant wavelengths of light. Fiber gratings are useful as, for example, filters for wavelength division multiplexing (WDM), gain flattening filters for optical amplifiers, and stabilizers for laser diodes used to pump optical amplifiers.
Typically, fiber gratings are made by laterally exposing the core of a single-mode fiber to a periodic pattern of intense actinic radiation (e.g., ultraviolet light). The exposure produces a permanent increase in the refractive index of the fiber's core, creating a fixed index modulation according to the exposure pattern. This fixed index modulation is called a grating. At each periodic refraction change, a small amount of light is reflected. All the reflected light signals combine coherently to one large reflection at a particular wavelength when the grating period is approximately half the input light's wavelength. This is referred to as the Bragg condition, and the wavelength at which this reflection occurs is called the Bragg wavelength.
In order to laterally expose the core of a fiber to form a grating, the fiber is typically moved relative to the light source (or vice versa). A challenge in the fabrication of gratings is to minimize positional errors between the multiple exposures in a grating caused by the relative movement between the fiber and the light source. Positional errors can result in phase errors between the multiple segments that form the grating. One approach to addressing this issue involves moving the fiber at a constant speed on a rotary stage while simultaneously modulating the write beams with a time-dependent function generator. This approach requires the rotary stage to maintain a constant angular velocity from which the rotary stage position is inferred. However, if the rotary stage is not maintained at a constant angular velocity, positional and phase errors are likely to occur.
A second approach involves carrying the fiber on a linear stage and stopping and exposing the fiber at various predetermined fixed locations along the fiber to stitch together a grating. While the stage carrying the fiber need not maintain a constant velocity with this approach, the stitching together of grating sections must be accomplished with an extremely high positional accuracy. Failure to maintain this accuracy also results in positional and phase errors in the fabrication process.
A third approach is a combination of the first two approaches and involves accurately knowing the position of the stage carrying the fiber as it moves the fiber with respect to an optical interference pattern. The stage position is read and the write beams are modulated according to the measured position. In this approach, a constant velocity is maintained by the stage, and the position of the stage must be constantly known to high accuracy. However, this approach only addresses errors due to fluctuations in the velocity of the stage carrying the fiber. Other perturbations common in a manufacturing environment, such as interference fringe drift or movement, vibrations of optical mounts, wavelength fluctuations, fluctuations of the write beam position, non-linearity of fiber photosensitivity, and so on, cause equivalent position or velocity errors but remain undetected and uncorrected. These other sources of error can increase the overall error in the system by an order of magnitude or more.
Thus, there is a need for a system that addresses these and other sources of error in the fabrication of structures in an optical substrate.